Kernel mode NEON for XOR and RAID6

Quoted 6 lines · Dave Martin

Significant benchmarks on the boot path would be unacceptable, unless they are *fast* (and by fast, I mean fast on all platforms, not just fast on the fast platforms). If one second gets added onto the boot path for each optimised algorithm, that sounds like a fail. If all the benchmarks combined take one second in total, that's no quite as bad.

Quoted 7 lines

Maybe benchmarks could be time-bounded (i.e., see how much data we can chug though in X milliseconds) instead of size-bounded. This would avoid unreasonable slowdown on slow platforms, while avoiding trivially small benchmark payloads on faster platforms which may typically have a more complex architecture, bigger caches etc. which would cause them to take longer to reach saturated performance when running a particular algorithm.

Benchmarks are already time bounded, at least the instances I am aware of (xor and raid6) are. They each measure, for each available implementation, the amount of work performed during a fixed time. For RAID6, this is 16 jiffies, for XOR it's only 1 jiffy but each test is repeated 5 times.

So I think this should not be a problem, especially as it is unlikely that newly added implementations (such as NEON) will be able to execute on older/slower platforms in the first place.